Process for Preparing Cyclic Esters and Cyclic Amides

ABSTRACT

A process for preparing a cyclic ester or a cyclic amide includes contacting hydroxycarboxylic acid and/or amino-carboxylic acid, an ester thereof, or a salt thereof, with acidic zeolite. The hydroxycarboxylic acid is a 2- or 6-hydroxycarboxylic acid, and the amino carboxylic acid is a 2- or 6-amino-carboxylic acid. The zeolite may include two or three interconnected and non-parallel channel systems, in which a channel system includes 10- or more-membered ring channels, and in which a framework Si/X 2  ratio is at least 24 as measured by NMR. X is Al or B. The zeolite may include three interconnected and non-parallel channel systems, in which at least two channel systems include 10- or more-membered ring channels, and in which a framework Si/X 2  ratio is at least 6 as measured by NMR. The process may be performed at a pressure between 0.5 and 20 bar.

FIELD OF THE INVENTION

The invention relates to a process for preparing cyclic esters andcyclic amides, which may be used as starting products for thepreparation of polymers such as biopolymers.

BACKGROUND OF THE INVENTION

Cyclic esters are useful compounds that can be polymerized intopolymeric materials. Such polymeric materials are useful in thepreparation of biodegradable plastic materials and other plasticmaterials. Cyclic esters are also useful as plasticizers and asintermediates for production of surface-active agents and plasticizers.

Cyclic esters are usually prepared by condensing hydroxy acids to anoligomeric prepolymer. The prepolymer is then depolymerized to a cyclicester. The production of a cyclic ester from an oligomeric prepolymer issometimes referred to as a back-biting reaction. The back-bitingreaction is typically a slow one, and a batch operation which extendsover significant time and which results in undesirable byproducts.Extensive purification processes are therefore required to obtain cyclicesters of requisite purity.

Therefore, there remains a need for processes for preparing cyclicesters that overcome one or more of the aforementioned issues. It is anobject of the present invention to provide a process for preparingcyclic esters and cyclic amides.

SUMMARY OF THE INVENTION

The present inventors have now surprisingly found that one or more ofthese objects can be obtained by the process of the present invention.

The present invention concerns a process for preparing a cyclic ester ora cyclic amide, comprising the step of contacting at least onehydroxycarboxylic acid and/or at least one amino-carboxylic acid; or anester, or salt thereof; wherein said hydroxycarboxylic acid is a2-hydroxycarboxylic acid or a 6-hydroxycarboxylic acid, and wherein saidamino carboxylic acid is a 2-amino-carboxylic acid or a6-amino-carboxylic acid;

with at least one acidic zeolite, wherein said zeolite comprises:

-   -   two or three interconnected and non-parallel channel systems        wherein at least one of said channel systems comprises 10- or        more-membered ring channels; and a framework Si/X₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems wherein at        least two of said channel systems comprise 10- or more-membered        ring channels, and a framework Si/X₂ ratio of at least 6 as        measured by NMR;        wherein each X is Al or B.

Preferably, the invention relates to a process for preparing a cyclicester or a cyclic amide, comprising the step of:

contacting at least one hydroxycarboxylic acid and/or at least oneamino-carboxylic acid; or an ester, or salt thereof; wherein saidhydroxycarboxylic acid is a 2-hydroxycarboxylic acid, or a6-hydroxycarboxylic acid; and wherein said amino carboxylic acid is a2-amino-carboxylic acid or a 6-amino-carboxylic acid;with at least one acidic zeolite comprising:

-   -   two or three interconnected and non-parallel channel systems,        wherein at least one of said channel systems comprises 10- or        more-membered ring channels; and a framework Si/X₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems, wherein        at least two of said channel systems comprise 10- or        more-membered ring channels; and a framework Si/X₂ ratio of at        least 6 as measured by NMR;        wherein each X is Al or B, and wherein the process is performed        at a pressure between 0.5 and 20 bar.

The independent and dependent claims set out particular and preferredfeatures of the invention. Features from the dependent claims may becombined with features of the independent or other dependent claims asappropriate.

The above and other characteristics, features and advantages of thepresent invention will become apparent from the following detaileddescription, which illustrates, by way of example, the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description of the figures of specific embodiments of theinvention is merely exemplary in nature and is not intended to limit thepresent teachings, their application or uses. Throughout the drawings,corresponding reference numerals indicate like or corresponding partsand features.

FIG. 1 Graph plotting the relative amount of lactic acid oligomers,lactide and lactic acid in a reactor at different times, using H-BEAzeolite catalyst with a Si/Al₂ ratio of 25.

FIG. 2 A, B: Graph plotting the rate of lactide production per acid siteper hour and per gram zeolite per hour, respectively, for ZSM-5 andH-BEA zeolites with varying Si/Al₂ ratios.

FIG. 3 Graph plotting the relative amounts of reaction products obtainedwith a H-BEA zeolite with a Si/Al₂ ratio of 25 for the synthesis oflactide, in various solvents.

FIG. 4 Graph plotting the relative amounts of reaction products obtainedwith H-BEA zeolites with Si/Al₂ ratios of 25 and 150 for the synthesisof ethyl glycolide from 2-hydroxybutanoic acid.

DETAILED DESCRIPTION OF THE INVENTION

When describing the processes of the invention, the terms used are to beconstrued in accordance with the following definitions, unless a contextdictates otherwise.

As used herein, the singular forms “a”, “an”, and “the” include bothsingular and plural referents unless the context clearly dictatesotherwise.

The terms “comprising”, “comprises” and “comprised of” as used hereinare synonymous with “including”, “includes” or “containing”, “contains”,and are inclusive or open-ended and do not exclude additional,non-recited members, elements or method steps. The terms “comprising”,“comprises” and “comprised of” also include the term “consisting of”.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to a person skilled in the art from this disclosure, in one ormore embodiments. Furthermore, while some embodiments described hereininclude some but not other features included in other embodiments,combinations of features of different embodiments are meant to be withinthe scope of the invention, and form different embodiments, as would beunderstood by those in the art. For example, in the following claims,any of the claimed embodiments can be used in any combination.

The recitation of numerical ranges by endpoints includes all numbers andfractions subsumed within the respective ranges, as well as the recitedendpoints.

Unless otherwise defined, all terms used in disclosing the invention,including technical and scientific terms, have the meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. By means of further guidance, definitions for the terms used inthe description are included to better appreciate the teaching of thepresent invention. The terms or definitions used herein are providedsolely to aid in the understanding of the invention.

All documents cited in the present specification are hereby incorporatedby reference in their entirety.

The present process for preparing a cyclic ester or a cyclic amide,comprises the step of contacting at least one hydroxycarboxylic acidand/or at least one amino-carboxylic acid; or an ester, or salt thereof;as defined herein; with at least one acidic zeolite as defined herein.

Preferably, the present process for preparing a cyclic ester or a cyclicamide, comprises the step of contacting at least one hydroxycarboxylicacid and/or at least one amino-carboxylic acid; or an ester, or saltthereof; as defined herein; with at least one acidic zeolite as definedherein, and the process is performed at a pressure between 0.5 and 20bar.

The term “zeolite” as used herein refers to both natural and syntheticmicroporous crystalline aluminosilicate materials having a definitecrystalline structure as determined by X-ray diffraction. A zeolitecomprises a system of channels which may be interconnected with otherchannel systems or cavities such as side-pockets or cages. The channelsystems may be three-dimensional, two-dimensional or one-dimensional. Azeolite comprises SiO₄ and XO₄ tetrahedra, wherein X is Al (aluminium)or B (boron). A zeolite may comprise a combination of AlO₄ and BO₄tetrahedra. In a preferred embodiment, X is Al, and the zeolitecomprises no BO₄ tetrahedra. The SiO₄ and XO₄ tetrahedra are linked attheir corners via a common oxygen atom. The Atlas of Zeolite FrameworkTypes (C Baerlocher, LB McCusker, DH Olson, 6^(th) ed. Elsevier,Amsterdam, 2007) in conjunction with the web-based version(http://www.iza-structure.org/databases/”) is a compendium oftopological and structural details about zeolite frameworks, includingthe types of ring structures present in the zeolite and the dimensionsof the channels defined by each ring type. Proven recipes and goodlaboratory practice for the synthesis of zeolites can be found in the“Verified synthesis of zeolitic materials” 2^(nd) Edition 2001. Variousproven recipes for the synthesis comprising BO₄ tetrahedra areavailable. For example, the synthesis and characterization ofboron-based zeolites having a MFI topology has been described byCichocki and Parasiewicz-Kaczmarska (Zeolites 1990, 10, 577-582).

Suitable zeolites for use in the present process typically comprise:

-   -   at least two, preferably two or three, interconnected and        non-parallel channel systems wherein at least one of said        channel systems comprises 10- or more-membered ring channels;        and a framework Si/X₂ ratio of at least 24 as measured by NMR;        or    -   three interconnected and non-parallel channel systems wherein at        least two of said channel systems comprise 10- or more-membered        ring channels, and a framework Si/X₂ ratio of at least 6 as        measured by NMR;        wherein each X is Al or B.

As used herein, the term “channel system” refers to a system of paralleland crystallographically equivalent channels, wherein the channels are8-membered ring channels or larger, for example wherein the channels are10-membered ring channels or 12-membered ring channels. Accordingly, asused herein, the term “channel” refers to an 8- or more membered ringchannel which is part of a system of parallel and crystallographicallyequivalent channels.

Suitable zeolites for use in the present process comprise 10- ormore-membered ring channels, such as 10-membered ring channels (10MR),12-membered ring channels (12MR), or larger. The ring size for eachknown zeolite framework type is provided in the Atlas of ZeoliteFramework Types (C Baerlocher, LB McCusker, DH Olson, 6^(th) ed.Elsevier, Amsterdam, 2007), which is incorporated herein by reference.

As used herein the terms “8-membered ring channels” or “8MR” refer to achannel comprising unobstructed 8-membered rings, wherein the 8-memberedrings define the smallest diameter of the channel. An 8-membered ringcomprises 8 T atoms, and 8 alternating oxygen atoms (forming the ring),wherein each T is Si, Al or B. As used herein the terms “10-memberedring channels” or “10MR” refers to a channel comprising unobstructed10-membered rings, wherein the 10-membered rings define the smallestdiameter of the channel. A 10-membered ring comprises 10 T atoms, and 10alternating oxygen atoms (forming the ring), wherein each T is Si, Al orB. As used herein the terms “12-membered ring channels” or “12MR” refersto a channel comprising unobstructed 12-membered rings, wherein the12-membered rings define the smallest diameter of the channel. A12-membered ring comprises 12 T atoms, and 12 alternating oxygen atoms(forming the ring), wherein each T is Si, Al or B. As used herein, theterm “10- or-more-membered ring channel” refers to a 10-membered ringchannel or larger, and therefore comprises for example both 10-memberedring channels and 12-membered ring channels.

The framework Si/X₂ ratio may be determined via Nuclear MagneticResonance (NMR) measurements, more particularly ²⁹Si and ²⁷Al NMR. In apreferred embodiment, there is no framework B, and the Si/X₂ ratio isequal to the Si/Al₂ ratio. The determination of the Si/Al₂ ratio by NMRmay be performed as described by Klinowski (Ann. Rev. Mater. Sci. 1988,18, 189-218); or as described by G. Engelhardt and D. Michel(High-Resolution Solid-State NMR of Silicates and Zeolites. John Wiley &Sons, Chichester 1987. xiv, 485 pp). The determination of the Si/B₂ratio by NMR may be performed as discussed by D. Trong On et al.(Studies in Surface Science and Catalysis 1995, 97, 535-541; Journal ofCatalysis, November 1995, Volume 157, Issue 1, Pages 235-243).

The present inventors have found that, by using certain zeolites ascatalysts, cyclic ester synthesis or cyclic amide synthesis can besimplified significantly. The present inventors have found that the useof zeolites as defined herein allow for the production of cyclic esterssuch as lactide in a single step, thus avoiding the condensation to aprepolymer and depolymerization towards cyclic ester. Moreover, theinventors found that zeolites as defined herein provide an excellentselectivity to the cyclic ester. Additionally, in contrast with thecondensation-depolymerization process, no significant racemizationoccurs when using zeolites as a catalyst, thus avoiding the extensivepurification. Zeolites are heterogeneous catalysts, and are thereforeeasy to separate from the product after reaction, in contrast withclassic homogeneous catalysts such as sulfuric acid.

The present process comprises the step of contacting at least onehydroxycarboxylic acid and/or at least one amino-carboxylic acid; or anester, or salt thereof, with at least one acidic zeolite, wherein saidzeolite comprises:

-   -   at least two, preferably two or three, interconnected and        non-parallel channel systems, wherein at least one of the        channel systems comprises 10- or more-membered ring channels;        and a framework Si/X₂ ratio of at least 24 as measured by NMR;        or    -   three interconnected and non-parallel channel systems wherein at        least two of the channel systems comprise 10- or more-membered        ring channels; and a framework Si/X₂ ratio of at least 6 as        measured by NMR, wherein each X is Al or B,        and wherein said hydroxycarboxylic acid is a 2-hydroxycarboxylic        acid or a 6-hydroxycarboxylic acid, and wherein said amino        carboxylic acid is a 2-amino-carboxylic acid or a        6-amino-carboxylic acid,        preferably wherein “channel system” refers to a system of        parallel and crystallographically equivalent channels, wherein        the channels are 8-membered ring channels or larger.

Indeed, the inventors have found that the selectivity towards cyclicesters or amides highly depends on the zeolite architecture. It wasfound that the best results were obtained using zeolites comprising atleast two interconnected and non-parallel channel systems (a 2D or 3Dmicropore geometry). Accordingly, the zeolites used in the processdescribed herein comprise a 2D or 3D micropore geometry, moreparticularly an interconnected 2D or 3D micropore geometry.

Furthermore, zeolites suitable for the process described herein havechannels which are large enough to accommodate the catalysis of thereaction from hydroxycarboxylic acid molecules to the respective cyclicester. The inventors found that the best results were obtained withzeolites comprising at least one 10- or more-membered ring channel.

The present inventors have further found that the Si/X₂ ratio in thezeolite framework significantly influences the suitability of thezeolites for catalyzing the reaction of hydroxycarboxylic acids tocyclic esters.

Accordingly, in particular embodiments, the zeolite(s) for use in theprocess described herein may comprise a framework Si/X₂ ratio of atleast 24, for example a framework Si/Al₂ ratio of at least 24, whereinthe zeolite further comprises at least two interconnected andnon-parallel channel systems wherein at least one of the interconnectedand non-parallel channel systems comprises 10- or more-membered ringchannels, i.e. at least one of the channel systems comprises 10- ormore-membered ring channels, and at least one other channel systemcomprises 8- or more-membered ring channels. Examples of such zeolitesare zeolites comprising a topology selected from the group comprisingFER, MFI, and MWW.

In yet further embodiments, both of the at least two channel systemscomprise 10- or more-membered ring channels. In particular embodiments,at least one of the channel systems comprises 12- or more-membered ringchannels.

In certain embodiments, the zeolite for use in the process describedherein may comprise a framework Si/X₂ ratio of at least 6, for example aframework Si/Al₂ ratio of at least 6; wherein the zeolite furthercomprises three interconnected and non-parallel channel systems whereinat least two of the interconnected and non-parallel channel systemscomprise 10- or more-membered ring channels, i.e. at least two of thechannel systems comprise 10- or more-membered ring channels, and theother channel system comprises 8- or more-membered ring channels.Examples of such zeolites include, but are not limited to zeolitescomprising a topology selected from the group comprising BEA, FAU, andMEL.

In yet further embodiments, the three channel systems all comprise 10-or more-membered ring channels. In particular embodiments, at least oneof the channel systems comprises 12- or more-membered channels. Incertain embodiments, at least two of the channel systems comprise 12- ormore-membered ring channels. Examples of such zeolites include, but arenot limited to zeolites comprising a topology selected from the groupcomprising BEA and FAU.

In particular embodiments, the zeolite comprises at least twointerconnected and non-parallel channel systems wherein at least one ofthe interconnected and non-parallel channel systems comprises 10- ormore-membered ring channels; wherein the zeolite further comprises aframework Si/X₂ ratio of at least 24, more particularly of at least 25,for example a ratio of at least 30, for example a ratio of at least 35,for example a ratio of at least 40, for example a ratio of at least 50,for example a ratio of at least 60, for example a ratio of at least 70,for example a ratio of at least 80, for example a ratio of at least 90,for example or at least 100. Preferably, the zeolite comprises two orthree interconnected and non-parallel channel systems wherein at leastone of the interconnected and non-parallel channel systems comprises 10-or more-membered ring channels; wherein the zeolite further comprises aframework Si/Al₂ ratio of at least 24, more particularly a ratio of atleast 25, for example a ratio of at least 30, for example a ratio of atleast 35, for example a ratio of at least 40, for example a ratio of atleast 50, for example a ratio of at least 60, for example a ratio of atleast 70, for example a ratio of at least 80, for example a ratio of atleast 90, or for example a ratio of at least 100.

In particular embodiments, the zeolite comprises three interconnectedand non-parallel channel systems wherein at least two of theinterconnected and non-parallel channel systems comprise 10- ormore-membered ring channels; wherein the zeolite further comprises aframework Si/X₂ ratio of at least 6, more particularly at least 8, forexample a ratio of at least 10, for example a ratio of at least 15, forexample a ratio of at least 20, for example a ratio of at least 25, forexample a ratio of at least 30, for example a ratio of at least 35, forexample a ratio of at least 40, for example a ratio of at least 50, forexample a ratio of at least 60, for example a ratio of at least 70, forexample a ratio of at least 80, for example a ratio of at least 90, orfor example a ratio of at least 100. Preferably, the zeolite comprisesthree interconnected and non-parallel channel systems wherein at leasttwo of the interconnected and non-parallel channel systems comprise 10-or more-membered ring channels; wherein the zeolite further comprises aframework Si/Al₂ ratio of at least 6, more particularly of at least 8,for example a ratio of at least 10, for example a ratio of at least 15,for example a ratio of at least 20, for example a ratio of at least 25,for example a ratio of at least 30, for example a ratio of at least 35,for example a ratio of at least 40, for example a ratio of at least 50,for example a ratio of at least 60, for example a ratio of at least 70,for example a ratio of at least 80, for example a ratio of at least 90,or for example a ratio of at least 100.

In most embodiments, the conversion of hydroxycarboxylic acids and/oraminocarboxylic acids to cyclic esters or cyclic amides increases as theSi/X₂ ratio increases, preferably as the Si/Al₂ ratio increases. In someembodiments, it is observed that at high Si/X₂ ratios, the conversionmay decrease as the Si/X₂ ratio increases further. Without wishing to bebound by theory, this is believed to be related to the low amount ofacid sites in zeolites with high Si/X₂ ratio. Therefore, in particularembodiments, the zeolite has a framework Si/X₂ ratio below 280. Infurther embodiments, the zeolite has a framework Si/X₂ ratio below 200.Preferably, the zeolite has a framework Si/Al₂ ratio below 280. Infurther embodiments, the zeolite has a framework Si/Al₂ ratio below 200.

The zeolites used in the process described herein may comprise AlO₄tetrahedra, BO₄ tetrahedra, or both. Accordingly, in some embodiments,X₂ is (Al₂+B₂). Thus, for a given zeolite, the Si/X₂ framework ratioremains the same upon substitution of framework Al by B, or vice versa.However, it is envisaged that in particular embodiments, the zeolitesmay not comprise BO₄ tetrahedra, or an insignificant amount thereof(e.g. an Al/B ratio of 100 or more). Thus, in particular embodiments, X₂may be Al₂.

The Si/X₂ ratios referred to herein are molar ratios as determined viaNMR, unless specified otherwise. It will be understood by the skilledperson that the Si/X₂ ratio referred to herein is equal to the SiO₂/X₂O₃molar ratio, wherein X₂O₃ is (Al₂O₃ and/or B₂O₃). Moreover, the skilledperson will understand that by dividing the Si/X₂ ratio by two, the Si/Xmolar ratio is obtained, wherein X is (Al and/or B).

Preferably, the channels defined by the zeolite topology are largeenough to be accessible for the monomers, but small enough to preventsignificant formation and/or diffusion of trimers or higher orderoligomers. Accordingly, in particular embodiments, the zeolite onlycomprises channels with a ring size of at most 18, preferably of at most14, for example of at most 12.

In a preferred embodiment, suitable zeolites for use in the processdescribed herein comprises a topology selected from the group comprisingBEA, MFI, FAU, MEL, FER, and MWW. The inventors have found that thesezeolites provide a particularly high selectivity towards cyclic esters.In certain embodiments, the zeolite(s) comprise a topology selected fromthe group consisting of BEA, MFI, FAU, and MWW. In specific embodiments,the zeolite(s) comprise a zeolite with a BEA topology.

Exemplary commercially available zeolites suitable for use in theprocesses described herein include, but are not limited to, Betapolymorph A (BEA topology), ZSM-5 (Mobil; MFI topology), Y zeolite (FAUtopology), and MCM-22 (Mobil; MWW topology).

In certain embodiments, the zeolite comprises channels having an average(equivalent) diameter of at least 4.5 Å. More particularly, the zeolitemay comprise two or more non-parallel channels having an averagediameter of at least 4.5 Å. The channel diameter may be determinedtheoretically via knowledge of the zeolite framework type, or via x-raydiffraction (XRD) measurements, as will be known by the skilled person.Preferably, the zeolite comprises two or more non-parallel andinterconnected channels having an average (equivalent) diameter between4.5 and 13.0 Å, more preferably between 4.5 and 8.5 Å. Preferably, thediameter for the appropriate topology is obtained from internationalstandard literature: the Atlas of Zeolite structures or thecorresponding online database, found athttp://www.iza-structure.org/databases/, as referenced above. The(equivalent) diameter of the channels may also be determinedexperimentally via N₂ adsorption, for example as discussed by Groen etal. (Microporous and Mesoporous Materials 2003, 60, 1-17), Storck et al.(Applied Catalysis A: General 1998, 174, 137-146) and Rouquerol et al.(Rouquerol F, Rouquerol J and Sing K, Adsorption by powders and poroussolids: principles, methodology and applications, Academic Press,London, 1999).

In some embodiments, the zeolite may further comprise mesopores. Thepresence of mesopores may increase the accessibility of thehydroxycarboxylic acids to the micropores, and may therefore furtherincrease the reaction speed. However, it is also envisaged that thezeolite may not comprise mesopores.

As used herein the term “mesopores” refers to pores in the zeolitecrystal having average diameters of 2.0 nm to 50 nm. For pore shapesdeviating from the cylinder, the above ranges of diameter of mesoporesrefer to equivalent cylindrical pores. The mesopore average diameter maybe determined by gas sorption techniques such as N₂ adsorption.

The zeolite(s) may be used as such, for example as a powder. In certainembodiments, the zeolite(s) may be formulated into a catalyst bycombining with other materials that provide additional hardness orcatalytic activity to the finished catalyst product. Materials which canbe blended with the zeolite may be various inert or catalytically activematerials, or various binder materials. These materials includecompositions such as kaolin and other clays, phosphates, alumina oralumina sol, titania, metal oxide such as zirconia, quartz, silica orsilica sol, metal silicates, and mixtures thereof. These components areeffective in densifying the catalyst and increasing the strength of theformulated catalyst. Various forms of rare earth metals can also beadded to the catalyst formulation. The catalyst may be formulated intopellets, spheres, extruded into other shapes, or formed into spray-driedparticles. The amount of zeolite which is contained in the finalcatalyst product may range from 0.5 to 99.9 weight %, preferably from2.5 to 99.5 weight % of the total catalyst, preferably from 2.5 to 95weight %, preferably from 2.5 to 90 weight % of the total catalyst, mostpreferably from 2.5 to 80 weight %; for example from 20 to 95 weight %,preferably from 20 to 90 weight %, most preferably from 20 to 80 weight%, with weight % based on the total weight of catalyst product.

In some embodiments, the zeolite(s) for use in the processes describedherein can be exposed to a (post-synthesis) treatment to increase theSi/X₂ framework ratio. Methods to increase the Si/Al₂ ratio of zeolitesare known in the art, and include dealumination of the framework via(hydro)thermal treatment, extraction of framework aluminum with acid,and replacement of framework aluminum with silicon by reaction withsilicon halides or hexafluorosilicates. An exemplary method ofdealumination is described by Remy et al. (J. Phys. Chem. 1996, 100,12440-12447; hereby incorporated by reference).

The zeolites for use in the process described herein preferably areBrønsted acidic zeolites, i.e. having proton donating sites in themicropores. In some embodiments, the zeolite has a Brønsted acid densitybetween 0.05 and 6.5 mmol/g dry weight. When all Al T-sites arecounterbalanced with an acidic proton (as opposed to a cation), theBrønsted acid density can be directly derived from the Si/Al₂ ratio, forexample as discussed in the Handbook of Heterogeneous Catalysis, secondedition, edited by G. Ertl, H. Knözinger, F. Schüth and J. Weitkamp,Wiley 2008.

The zeolites for use in the processes described herein can be obtainedin acidic form (acidic H-form zeolite) or (partly) exchanged with acation other than H⁺. In some embodiments, the acidic H-form zeolitescan be used as such. In some other embodiments, the zeolites for use inthe processes described herein can be exposed to a (post-synthesis)treatment to increase the Brønsted acid density. Brønsted acid sites inzeolites can be readily generated by aqueous ion exchange with anammonium salt, followed by thermal decomposition of the ammonium ionsinside the zeolite. Alternatively, the acid sites may be generated byaqueous ion exchange with the salt of a multivalent metal cation (suchas Mg²⁺, Ca²⁺, La²⁺, or mixed rare-earth cations), followed by thermaldehydration (J. Weitkamp, Solid State Ionics 2000, 131, 175-188; herebyincorporated by reference).

In contrast with polymeric catalysts (e.g. Amberlyst™), the zeolitecatalysts described herein may be regenerated and reused in the process.Accordingly, particular embodiments of the process described herein maycomprise a step of regenerating the zeolite catalyst. Regeneration ofthe zeolite catalysts can be performed via washing or calcination.Preferably, regeneration of the zeolite catalysts is done viacalcination, for example at a temperature of at least 150° C. Inparticular embodiments, the calcination temperature is at least 200° C.,for example at least 300° C., for example at least 400° C., for exampleabout 450° C.

In the processes described herein, at least one hydroxycarboxylic acid,and/or at least one aminocarboxylic acid are used as starting material.

The hydroxycarboxylic acid used in the context of the processesdescribed herein is selected from a 2-hydroxycarboxylic acid or a6-hydroxycarboxylic acid. Also salts, or esters of such compounds may beused. In particular embodiments, only one hydroxycarboxylic acid is usedin the process. In some embodiments, it is envisaged that two differenthydroxycarboxylic acids can be used, for example for the preparation ofasymmetric dimeric cyclic esters. In particular embodiments, said2-hydroxycarboxylic acid comprises at least 3 carbon atoms.

In some embodiments, the zeolite is contacted with at least one compoundof formula (I)

or a salt, or ester thereof;wherein

R⁵ is OH or NH₂; and

R¹ and R² are each independently hydrogen or a group selected fromC₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy. In further embodiments, R¹ and R² are each independentlyselected from hydrogen or a group selected from C₁₋₄alkyl, C₂₋₄alkenyl,or C₂₋₄alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; preferably R¹ and R²are each independently selected from hydrogen, C₁₋₆alkyl, C₂₋₆alkenyl,C₆₋₁₀aryl, C₁₋₁₀alkylC₆₋₁₀arylene, C₆₋₁₀arylC₁₋₆alkylene, orC₂₋₆alkynyl. In further embodiments, R¹ and R² are each independentlyselected from hydrogen, C₁₋₄alkyl, or C₂₋₄alkenyl.

In particular embodiments, at least one of R¹ and R² is not hydrogen.Unless expressly stated otherwise, each of the following terms has theindicated meaning:

The term “C₁₋₆alkyl”, as a group or part of a group, refers to ahydrocarbyl radical of Formula C_(n)H_(2n+1) wherein n is a numberranging from 1 to 6. Generally, the alkyl groups comprise from 1 to 6carbon atoms, for example 1 to 4 carbon atoms. Alkyl groups may belinear, or branched and may be substituted as indicated herein. When asubscript is used herein following a carbon atom, the subscript refersto the number of carbon atoms that the named group may contain. Thus,for example, C₁₋₄alkyl means an alkyl of 1 to 4 carbon atoms. Examplesof alkyl groups are methyl, ethyl, propyl, isopropyl, butyl, isobutyl,sec-butyl, tert-butyl, pentyl and its chain isomers, hexyl and its chainisomers.

The term “C₂₋₆alkenyl” by itself or as part of another substituent,refers to an unsaturated hydrocarbyl group, which may be linear, orbranched, comprising one or more carbon-carbon double bonds. Preferredalkenyl groups thus comprise between 2 and 6 carbon atoms, preferablybetween 2 and 4 carbon atoms. Non-limiting examples of C₂₋₆alkenylgroups include ethenyl, 2-propenyl, 2-butenyl, 3-butenyl, 2-pentenyl andits chain isomers, 2-hexenyl and its chain isomers, 2,4-pentadienyl andthe like.

The term “C₂₋₆alkynyl” by itself or as part of another substituent,refers to an unsaturated hydrocarbyl group, which may be linear, orbranched, comprising one or more carbon-carbon triple bonds. Preferredalkynyl groups thus comprise between 2 and 6 carbon atoms, preferablybetween 2 and 4 carbon atoms. Non limiting examples of C₂₋₆alkynylgroups include ethynyl, 2-propynyl, 2-butynyl, 3-butynyl, 2-pentynyl andits chain isomers, 2-hexynyl and its chain isomers and the like.

As used herein, the term “C₆₋₁₀aryl”, by itself or as part of anothersubstituent, refers to a polyunsaturated, aromatic hydrocarbyl grouphaving a single ring (i.e. phenyl) or multiple aromatic rings fusedtogether (e.g. naphthalene), or linked covalently, typically containing6 to 10 atoms; wherein at least one ring is aromatic. Examples ofC₆₋₁₀aryl include phenyl, naphthyl, indanyl, or1,2,3,4-tetrahydro-naphthyl.

The term “C₁₋₆alkoxy” or “C₁₋₆alkyloxy” as used herein refers to aradical having the Formula —OR^(d) wherein Rd is C₁₋₆alkyl. Non-limitingexamples of suitable alkoxy include methoxy, ethoxy, propoxy,isopropoxy, butoxy, isobutoxy, sec-butoxy, tert-butoxy, pentyloxy andhexyloxy.

As used herein, the term “C₁₋₆alkylene”, by itself or as part of anothersubstituent, refers to C₁₋₆alkyl groups that are divalent, i.e., withtwo single bonds for attachment to two other groups. Alkylene groups maybe linear or branched and may be substituted as indicated herein.Non-limiting examples of alkylene groups include methylene (—CH₂—),ethylene (—CH₂—CH₂—), methylmethylene (—CH(CH₃)—), 1-methyl-ethylene(—CH(CH₃)—CH₂—), n-propylene (—CH₂—CH₂—CH₂—), 2-methylpropylene(—CH₂—CH(CH₃)—CH₂—), 3-methylpropylene (—CH₂—CH₂—CH(CH₃)—), n-butylene(—CH₂—CH₂—CH₂—CH₂—), 2-methylbutylene (—CH₂—CH(CH₃)—CH₂—CH₂—),4-methylbutylene (—CH₂—CH₂—CH₂—CH(CH₃)—), pentylene and its chainisomers, hexylene and its chain isomers.

The term “C₆₋₁₀arylC₁₋₆alkylene”, as a group or part of a group, means aC₁₋₆alkyl as defined herein, wherein a hydrogen atom is replaced by aC₆₋₁₀aryl as defined herein. Examples of C₆₋₁₀arylC₁₋₆alkyl radicalsinclude benzyl, phenethyl, dibenzylmethyl, methylphenylmethyl,3-(2-naphthyl)-butyl, and the like.

As used herein, the term “C₁₋₆alkylC₆₋₁₀arylene”, by itself or as partof another substituent, refers to a C₆₋₁₀aryl group as defined herein,wherein a hydrogen atom is replaced by a C₁₋₆alkyl as defined herein.

In some embodiments, the hydroxycarboxylic acid is a 2-hydroxycarboxylicacid, more particularly a compound of formula (Ia)

or a salt, or ester thereof;wherein R¹ and R² are each independently hydrogen or a group selectedfrom C₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy. In further embodiments, R¹ and R² are each independentlyselected from hydrogen; or a group selected from C₁₋₄alkyl, C₂₋₄alkenyl,or C₂₋₄alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; preferably R¹ and R²are each independently selected from hydrogen, C₁₋₆alkyl, C₂₋₆alkenyl,C₆₋₁₀aryl, C₁₋₁₀alkylC₆₋₁₀arylene, C₆₋₁₀arylC₁₋₆alkylene, orC₂₋₆alkynyl. In further embodiments, R¹ and R² are each independentlyselected from hydrogen, C₁₋₄alkyl, or C₂₋₄alkenyl.

In certain embodiments, at least one of R¹ and R² is not hydrogen.

In certain embodiments, the 2-hydroxycarboxylic acid (also referred toas “α-hydroxycarboxylic acid”) is a compound of formula (Ia), wherein R¹is hydrogen and R² is a group selected from C₁₋₆alkyl; C₂₋₆alkenyl;C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene; C₆₋₁₀arylC₁₋₆alkylene; orC₂₋₆alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; preferably R² isselected from C₁₋₆alkyl; C₂₋₆alkenyl; or C₂₋₆alkynyl; preferably R² isC₁₋₄alkyl, or C₂₋₄alkenyl.

In preferred embodiments, the compound of formula (I) or (Ia) can beselected from the group comprising lactic acid, 2-hydroxybutanoic acid,2-hydroxy-3-butenoic acid, and 2-hydroxyhexanoic acid. Preferably thecompound of formula (I) or (Ia) is lactic acid.

In some embodiments, the hydroxycarboxylic acid is a 6-hydroxycarboxylicacid, for example 6-hydroxyhexanoic acid, optionally substituted withone or more groups selected from the group consisting of halo,C₁₋₄alkyl, C₂₋₄alkenyl, or C₂₋₄alkynyl. 6-hydroxycarboxylic acids areparticularly useful for the preparation of ε-lactones, such ascaprolactone.

Preferably, said hydroxycarboxylic acid is selected from the groupcomprising lactic acid, 2-hydroxybutanoic acid, 2-hydroxy-3-butenoicacid, 2-hydroxyhexanoic acid, 6-hydroxyhexanoic acid, and glycolic acid.Preferably, said hydroxycarboxylic acid is lactic acid.

In some embodiments, the aminocarboxylic acid is a 2-aminocarboxylicacid, more particularly a compound of formula (Ib)

or a salt, or ester thereof;wherein R¹ and R² are each independently hydrogen or a group selectedfrom C₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy. In further embodiments, R¹ and R² are each independentlyselected from hydrogen or a group selected from C₁₋₄alkyl, C₂₋₄alkenyl,or C₂₋₄alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; preferably R¹ and R²are each independently selected from hydrogen, C₁₋₆alkyl, C₂₋₆alkenyl,C₆₋₁₀aryl, C₁₋₁₀alkylC₆₋₁₀arylene, C₆₋₁₀arylC₁₋₆alkylene, orC₂₋₆alkynyl. In further embodiments, R¹ and R² are each independentlyselected from hydrogen, C₁₋₄alkyl, or C₂₋₄alkenyl.

In certain embodiments, at least one of R¹ and R² is not hydrogen.

In certain embodiments, the aminocarboxylic acid is a compound offormula (Ib), wherein R¹ is hydrogen and R² is a group selected fromC₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy; preferably R² is selected from C₁₋₆alkyl; C₂₋₆alkenyl; orC₂₋₆alkynyl; preferably R² is C₁₋₄alkyl, or C₂₋₄alkenyl.

In preferred embodiments, the compound of formula (I) or (Ib) can bealanine.

In some embodiments, the aminocarboxylic acid is a 6-aminocarboxylicacid, for example 6-aminohexanoic acid, optionally substituted with oneor more groups selected from the group consisting of halo, C₁₋₄alkyl,C₂₋₄alkenyl, or C₂₋₄alkynyl. 6-aminocarboxylic acids are particularlyuseful for the preparation of caprolactam and derivatives thereof.

The processes described herein may be used for the production of variouscyclic esters, or cyclic amides, such as dimeric cyclic esters,lactones, dimeric cyclic amides, or lactams.

In preferred embodiments, the cyclic ester or cyclic amide prepared bythe process described herein, is a compound of formula (II):

wherein

Z¹ is O or NH; Z² is O or NH;

R¹ and R² are each independently hydrogen or a group selected fromC₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy. In further embodiments, R¹ and R² are each independentlyselected from hydrogen or a group selected from C₁₋₄alkyl, C₂₋₄alkenyl,or C₂₋₄alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; preferably R¹ and R²are each independently selected from hydrogen, C₁₋₆alkyl, C₂₋₆alkenyl,C₆₋₁₀aryl, C₁₋₁₀alkylC₆₋₁₀arylene, C₆₋₁₀arylC₁₋₆alkylene, orC₂₋₆alkynyl. In further embodiments, R¹ and R² are each independentlyselected from hydrogen, C₁₋₄alkyl, or C₂₋₄alkenyl.

In certain embodiments, at least one of R¹ and R² is not hydrogen.

Compounds of formula (II) may be obtained by reaction of one or morecompounds of formula (I) as described herein.

In preferred embodiments, the cyclic ester or cyclic amide prepared bythe processes described herein is symmetric.

In some embodiments, it is also envisaged that the dimeric cyclic esteror cyclic amide may be asymmetric. For the preparation of asymmetriccompounds of formula (II), two different compounds of formula (I) arerequired.

Preferably, the present process, comprises the step of contacting atleast one compound of formula (I), a salt or an ester thereof,

with at least one acidic zeolite thereby obtaining a compound of formula(II),wherein said zeolite comprises:

-   -   at least two interconnected and non-parallel channel systems        wherein at least one of the channel systems comprises 10- or        more-membered ring channels; and a framework Si/X₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems wherein at        least two of the channel systems comprise 10- or more-membered        ring channels; and a framework Si/X₂ ratio of at least 6 as        measured by NMR;        wherein each X is Al or B.

Preferably, the present process, comprises the step of contacting atleast one compound of formula (I), a salt or an ester thereof,

with at least one acidic zeolite thereby obtaining a compound of formula(II),wherein said zeolite comprises:

-   -   two or three interconnected and non-parallel channel systems        wherein at least one of the channel systems comprises 10- or        more-membered ring channels; and a framework Si/Al₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems wherein at        least two of the channel systems comprise 10- or more-membered        ring channels; and a framework Si/Al₂ ratio of at least 6 as        measured by NMR.

In preferred embodiments, the cyclic ester prepared by the processdescribed herein, is a compound of formula (IIa):

wherein R¹ and R² are each independently hydrogen or a group selectedfrom C₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy; and wherein at least one of R¹ and R² is not hydrogen. Infurther embodiments, R¹ and R² are each independently selected fromhydrogen or a group selected from C₁₋₄alkyl, C₂₋₄alkenyl, orC₂₋₄alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; and wherein at leastone of R¹ and R² is not hydrogen; preferably R¹ and R² are eachindependently selected from hydrogen, C₁₋₆alkyl, C₂₋₆alkenyl, C₆₋₁₀aryl,C₁₋₁₀alkylC₆₋₁₀arylene, C₆₋₁₀arylC₁₋₆alkylene, or C₂₋₆alkynyl; andwherein at least one of R¹ and R² is not hydrogen. In furtherembodiments, R¹ and R² are each independently selected from hydrogen,C₁₋₄alkyl, or C₂₋₄alkenyl, wherein at least one of R¹ and R² is nothydrogen.

Compounds of formula (IIa) may be obtained by reaction of one or morecompounds of formula (Ia) as described herein.

In certain embodiments, the hydroxycarboxylic acid is a compound offormula (IIa), wherein R¹ is hydrogen and R² is a group selected fromC₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy; preferably R² is selected from C₁₋₆alkyl; C₂₋₆alkenyl; orC₂₋₆alkynyl; preferably R² is C₁₋₄alkyl, or C₂₋₄alkenyl.

In preferred embodiments, the compound of formula (IIa) prepared by theprocesses described herein is symmetric.

In some embodiments, it is also envisaged that the compound of formula(IIa) may be asymmetric. For the preparation of asymmetric compounds offormula (IIa), two different compounds of formula (Ia) are required.

In some embodiments, the cyclic ester is a lactone, more particularly aε-lactone, which may be prepared by using a 6-hydroxycarboxylic acid ashydroxyhexanoic acid.

Preferably, the process for preparing a cyclic ester, comprises the stepof contacting at least one compound of formula (Ia), with at least oneacidic zeolite thereby obtaining a compound of formula (IIa),

wherein said zeolite comprises:

-   -   at least two interconnected and non-parallel channel systems        wherein at least one of the channel systems comprises 10- or        more-membered ring channels; and a framework Si/X₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems wherein at        least two of the channel systems comprise 10- or more-membered        ring channels; and a framework Si/X₂ ratio of at least 6 as        measured by NMR,        wherein each X is Al or B.

More preferably, the process for preparing a cyclic ester, comprises thestep of contacting at least one compound of formula (Ia), with at leastone acidic zeolite thereby obtaining a compound of formula (IIa),

wherein said zeolite comprises:

-   -   two or three interconnected and non-parallel channel systems        wherein at least one of the channel systems comprises 10- or        more-membered ring channels; and a framework Si/Al₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems wherein at        least two of the channel systems comprise 10- or more-membered        ring channels; and a framework Si/Al₂ ratio of at least 6 as        measured by NMR.

In a preferred embodiment, lactic acid is used in the presence of atleast one zeolite as defined herein to produce lactide. Preferably,L-lactic acid is used in the presence of at least one zeolite as definedherein to produce L-L-lactide. Preferably, D-lactic acid is used in thepresence of at least one zeolite as defined herein to produceD-D-lactide.

In some embodiments, the cyclic amide prepared by the process describedherein, is a compound of formula (IIb):

wherein R¹ and R² are each independently hydrogen or a group selectedfrom C₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy. In further embodiments, R¹ and R² are each independentlyselected from hydrogen or a group selected from C₁₋₄alkyl, C₂₋₄alkenyl,or C₂₋₄alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy; preferably R¹ and R²are each independently selected from hydrogen, C₁₋₆alkyl, C₂₋₆alkenyl,C₆₋₁₀aryl, C₁₋₁₀alkylC₆₋₁₀arylene, C₆₋₁₀arylC₁₋₆alkylene, orC₂₋₆alkynyl. In further embodiments, R¹ and R² are each independentlyselected from hydrogen, C₁₋₄alkyl, or C₂₋₄alkenyl.

In particular embodiments, at least one of R¹ and R² is not hydrogen.

Compounds of formula (IIb) may be obtained by reaction of one or morecompounds of formula (Ib) as described herein.

In certain embodiments, the hydroxycarboxylic acid is a compound offormula (IIb), wherein R¹ is hydrogen and R² is a group selected fromC₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy; preferably R² is selected from C₁₋₆alkyl; C₂₋₆alkenyl; orC₂₋₆alkynyl; preferably R² is C₁₋₄alkyl, or C₂₋₄alkenyl.

In preferred embodiments, the compound of formula (IIb) prepared by theprocesses described herein is symmetric.

In some embodiments, it is also envisaged that the compound of formula(IIb) may be asymmetric. For the preparation of asymmetric compounds offormula (IIb), two different compounds of formula (Ib) are required.

In some embodiments, the cyclic amide is a lactam, more particularly aε-lactam, such as caprolactam, which may be prepared by using a6-aminocarboxylic acid as aminohexanoic acid.

Preferably, the process for preparing a cyclic amide, comprises the stepof contacting at least one compound of formula (Ib), with at least oneacidic zeolite thereby obtaining a compound of formula (IIb),

wherein said zeolite comprises:

-   -   two or three interconnected and non-parallel channel systems        wherein at least one of the channel systems comprises 10- or        more-membered ring channels; and a framework Si/X₂ ratio of at        least 24 as measured by NMR; or    -   three interconnected and non-parallel channel systems wherein at        least two of the channel systems comprise 10- or more-membered        ring channels; and a framework Si/X₂ ratio of at least 6 as        measured by NMR        wherein each X is Al or B.

The hydroxycarboxylic acid and/or aminocarboxylic acid is provided insolution or emulsion, preferably in solution.

An appropriate solvent may be one in which the reaction productsdescribed herein are soluble and which has an appropriate boiling point.More particularly, the boiling point preferably is sufficiently high sothat at the boiling point temperature an acceptable reaction rate isachieved, but sufficiently low such that the formation of degradationproducts can be avoided or minimized. Preferably, the solvent forms anazeotrope with water, thereby allowing the removal of water viaazeotropic distillation. Azeotropic solvents can include waterimmiscible aromatic solvents, water immiscible aliphatic or cyclichydrocarbon solvents, water soluble solvents, or mixtures thereof. Waterimmiscible azeotropic solvents are preferred because, afterdistillation, they can be readily separated with the solvent beingrecycled and the water being taken out of the system. Moreover,potential byproducts obtained during the reaction process (such as watersoluble short oligomers of the hydroxycarboxylic acid and/oraminocarboxylic acid) will typically dissolve in the water phase, whilethe cyclic esters and/or cyclic amides of interest typically remain inthe organic solvent phase. This may facilitate the separation of thebyproducts from the products of interest via extraction, and subsequentre-entry of the water soluble products (after hydrolysis) in thereaction process.

Solvents which are not preferred because of being potentially reactivewith cyclic esters include alcohols, organic acids, esters and etherscontaining alcohol, peroxide and/or acid impurities, ketones andaldehydes with a stable enol form, and amines.

Suitable solvents may include aromatic hydrocarbon solvents such asbenzene, toluene, xylene, ethylbenzene, trimethylbenzene (e.g.1,3,5-trimethylbenzene), methylethylbenzene, n-propylbenzene,isopropylbenzene, diethylbenzene, isobutylbenzene, triethylbenzene,diisopropylbenzene, n-amylnaphthalene, and trimethylbenzene; ethersolvents such as ethyl ether, isopropyl ether, n-butyl ether, n-hexylether, 2-ethylhexyl ether, ethylene oxide, 1,2-propylene oxide,dioxolane, 4-methyldioxolane, 1,4-dioxane, dimethyldioxane, ethyleneglycol diethyl ether, ethylene glycol dibutyl ether, diethylene glycoldiethyl ether, diethylene glycol di-n-butyl ether, tetrahydrofuran, and2-methyltetrahydrofuran; aliphatic hydrocarbon solvents such asn-pentane, isopentane, n-hexane, isohexane, n-heptane, isoheptane,2,2,4-trimethylpentane, n-octane, isooctane, cyclohexane, andmethylcyclohexane; and ketone solvents such as acetone, methyl ethylketone, methyl n-propyl ketone, methyl n-butyl ketone, diethyl ketone,methyl isobutyl ketone, methyl n-pentyl ketone, ethyl n-butyl ketone,methyl n-hexyl ketone, diisobutyl ketone, trimethylnonanone,cyclohexanone, 2-hexanone, methylcyclohexanone, 2,4-pentanedione,acetonylacetone, acetophenone, and fenchone.

Particularly preferred solvents include, but are not limited to toluene,ortho-xylene, meta-xylene, para-xylene, ethylbenzene, propylbenzene,trimethylbenzene, anisol, or mixtures thereof.

In the processes as described herein, the hydroxycarboxylic acid(s)and/or aminocarboxylic acid(s) may be provided in a composition, such asin a solvent or diluent, which comprises the hydroxycarboxylic acid(s)and/or aminocarboxylic acid(s) in high concentrations, for example inconcentrations of at least 1 weight % (wt %) based on the total weightof the composition, for example of at least 3 weight % (wt %) based onthe total weight of the composition, for example of at least 5 weight %(wt %) based on the total weight of the composition, for example of atleast 10 wt %, for example of at least 15 wt %, for example of at least20 wt %, for example of at least 25 wt %, for example of at least 30 wt%, for example of at least 35 wt %, for example of at least 40 wt %, forexample of at least 45 wt %, or for example of at least 50 wt % based onthe total weight of the composition. In preferred embodiments, thehydroxycarboxylic acid (or salt, or ester thereof) may be provided in acomposition comprising the hydroxycarboxylic acid (or salt, ester oramide thereof) in a concentration of at least 1 weight % based on thetotal weight of the composition. In further embodiments, the compositionmay comprise the hydroxycarboxylic acid, preferably lactic acid, in aconcentration of at least 5 wt %, for example of at least 5 wt %, forexample of at least 10 wt %, for example of at least 15 wt %, forexample of at least 20 wt %, for example of at least 25 wt %, forexample of at least 30 wt %, for example of at least 35 wt %, forexample of at least 40 wt %, for example of at least 45 wt %, or forexample of at least 50 wt % based on the total weight of thecomposition.

The process described herein is preferably performed under conditions ofwater removal. This may be obtained via a variety of methods, such asazeotropic distillation, evaporation, using molecular sieves or osmoticmembranes, using anhydrous salts that form hydrated crystals with water,and contacting the feedstream with water absorptive materials such aspolysaccharides or silica. Preferably azeotropic distillation is used.In preferred embodiments, the water is removed at least partially fromthe reaction mixture by azeotropic distillation, for example via aDean-Stark apparatus.

The processes described herein may be performed at or near atmosphericpressure, and are typically performed at a pressure between 0.5 and 20bar. In particular embodiments, the processes are performed at apressure between 0.5 and 5 bar, more particularly between 0.9 and 1.1bar.

The reaction may be performed at a relatively low temperature, and maytherefore require less energy than other processes known in the art. Inparticular embodiments, the reaction is performed at the boiling pointof the reaction mixture. In particular embodiments, the temperature ofthe heating device used for the reaction can be ranging from 50 to 300°C. In an embodiment, a higher temperature of the heating device willresult in an increased reflux power at the same boiling temperature ofthe reaction mixture.

In particular embodiments, pure isomeric forms of the hydroxycarboxylicacid or aminocarboxylic acid can be used. However, it is also envisagedthat in certain embodiments, a racemic mixture of the hydroxycarboxylicacid or aminocarboxylic acid can be used. Pure isomeric forms aredefined as isomers substantially free of other enantiomeric ordiastereomeric forms of the same basic molecular structure. Inparticular, the term “stereoisomerically pure” or “chirally pure”relates to compounds having a stereoisomeric excess of at least about80% (i.e. at least 90% of one isomer and at most 10% of the otherpossible isomers), preferably at least 90%, more preferably at least 94%and most preferably at least 97%. The terms “enantiomerically pure” and“diastereomerically pure” should be understood in a similar way, havingregard to the enantiomeric excess, respectively the diastereomericexcess, of the mixture in question.

The term “enantiomeric excess” or “% ee” as used herein refers to theamount of an enantiomer with respect to another. It can be calculated asfollows:

% ee=[([A]−[B]):([A]+[B]]×100,

wherein [A] is the concentration of one of the enantiomers, and [B] isthe concentration of the other enantiomer. The concentration of each ofthe enantiomers is, of course, expressed on the same basis, and can beexpressed on either a weight of molar basis because the enantiomers havethe same molecular weight.

Consequently, if a mixture of enantiomers is obtained during any of thefollowing preparation methods, it can be separated by liquidchromatography using a suitable chiral stationary phase. Suitable chiralstationary phases are, for example, polysaccharides, in particularcellulose or amylose derivatives. Commercially available polysaccharidebased chiral stationary phases are ChiralCel™ CA, OA, OB, OC, OD, OF,OG, OJ and OK, and Chiralpak™ AD, AS, OP(+) and OT(+). Appropriateeluents or mobile phases for use in combination with said polysaccharidechiral stationary phases are hexane and the like, modified with analcohol such as ethanol, isopropanol and the like.

The processes described herein may be therefore be used for thepreparation of enantiomerically pure cyclic esters. Indeed, theinventors found that during the process, no significant racemisationoccurs. This means that if enantiomerically pure starting products(hydroxycarboxylic acids) are used, the resulting cyclic esters willalso be enantiomerically pure, without further purification.Accordingly, in particular embodiments the hydroxycarboxylic acid(s)used in the processes described herein are enantiomerically pure, withan enantiomeric excess of at least 90%, preferably at least 95%.

Lactide has two asymmetric carbon atoms so it may be obtained in threestereoisomeric forms: L-L-lactide in which both asymmetric carbon atomspossess the L (or S) configuration; D-D-lactide in which both asymmetriccarbon atoms possess the D (or R) configuration; and meso-lactide(D-L-lactide) in which one asymmetric carbon atom has theL-configuration and the other has the D-configuration.

In certain embodiments of the processes described herein, thehydroxycarboxylic acid is L-lactic acid (with an enantiomeric excess ofat least 90%, preferably at least 95%, more preferably at least 98%) andthe corresponding cyclic ester is L-L-lactide.

In certain embodiments of the processes described herein, thehydroxycarboxylic acid is D-lactic acid (with an enantiomeric excess ofat least 90%, preferably of at least 95%, more preferably of at least98%) and the corresponding cyclic ester is D-D-lactide.

The invention will now be illustrated by the following, non-limitingillustrations of particular embodiments of the invention.

EXAMPLES Example 1 Preparation of Lactide from Lactic Acid

In this example several zeolites were tested as catalyst for thesynthesis of L-L-lactide from L-lactic acid.

The following zeolites were used: CBV500, CBV600, CBV720, CBV760, andCBV780 (available from Zeolyst International, in NH₄ or H-form); H-BEA(available from Süd-Chemie), NH₄-ZSM-5 with various Si/Al₂ ratios(available from Zeolyst International); H-MOR (available fromSüd-Chemie); H-FER (available from Zeolyst International); H-MCM-22(available from ACSMaterial); LaX and LaY (made by starting from NaY orNaX, available from Evonik, according to C. F. Heylen and P. A. Jacobs,(Advances in Chemistry Series, 1973, 121, 490-500)).

The zeolites were used in their Brønsted acidic form (H-form). Ingeneral, when zeolites were provided (partly) exchanged with othercations (such as Sodium cations), they were exchanged and calcined tomaximize the acidity and achieve the H-form. Typically, 100 mL of anaqueous solution of 0.5 M NH₄Cl was added per 1.0 gram of (e.g. Na)zeolite on wet basis. The mixture was heated for 4 hours under refluxconditions. Then, the zeolite was isolated by filtration and theexchange procedure was repeated. The zeolite was isolated again, andwashed with 1 L of water. In this way, the NH₄-form of the zeolite isobtained. To transform this ammonium exchanged form into the Brønstedacidic form, the zeolite was typically calcined for 12 hours at atemperature of 450° C. A temperature ramp of 3° C./min was applied. Theresulting zeolites were stored at room temperature in contact with air.

In a typical experiment, a reaction flask was loaded with a mixture ofabout 10 wt % L-lactic acid (L-LA) in toluene. Unless mentionedotherwise, the solution was prepared by mixing 1 g of 90 wt % L-LA(aqueous, obtained from Acros Organics) with 10 ml toluene. In oneexperiment (using the H-FER zeolite as catalyst), the solution wasprepared by mixing 1.65 g of 50 wt % L-LA (aqueous, obtained fromSigma-Aldrich) with 10 ml toluene. In the conditions used for theexperiments herein, no significant difference was observed between thesestarting solutions under these conditions.

The zeolite was added to the reaction mixture (about 0.5 g of zeoliteper 10 mL solution), and the mixture was heated by placing the reactionflask in a hot oil bath at a temperature of about 130° C., andcontinuously mixed, the temperature of the reaction mixture wasdependent on the used solvent and composition. A Dean-Stark trap wasused for removal of water from the reaction mixture. Typically, thereaction mixture was heated for about 3 hours under stirring, afterwhich the mixture was cooled to room temperature. The relative amountsof lactic acid oligomers, lactic acid, and lactide in the reactionmixture after 3 hours was indicative of the yield obtainable with eachcatalyst, as the reaction mixture typically does not changesignificantly after 3 hours for a good catalyst. This can be appreciatedfrom FIG. 1, which shows the relative amount of reaction products in areactor at different times, using a H-BEA zeolite catalyst with a Si/Al₂ratio of 25. However, it is noted that for some catalysts, the maximalconcentrations may be obtained faster.

Reference experiments were conducted using the known catalysts sulfuricacid (0.01 g per 10 mL solution) and Amberlyst® 15 Wet (about 0.5 g per10 mL solution). The amount of reference catalysts is chosen such thatthe total amount of acid sites is similar to the amount of acid sites ofthe zeolites, thus allowing a fair comparison.

For each experiment, the total conversion rate of the lactic acid, andthe lactide yield were determined via ¹H NMR. Also control measurementsusing gas chromatography with flame ionization detector (GC/FID) andhigh-pressure liquid chromatography (HPLC) with uv-visible detector wereperformed. The total conversion of the lactic acid includes the fractionof lactic acid which had reacted to lactide, trimers, or otheroligomers. The lactide yield only includes the fraction of fed lacticacid which has reacted to lactide.

All zeolites having two or three interconnected and non-parallel channelsystems, with at least one of said systems comprising 10- ormore-membered ring channels and a framework Si/Al₂ ratio of at least 24,and all zeolites having three interconnected and non-parallel channelsystems, with at least two of said channel systems comprising 10- ormore-membered ring channels and a framework Si/Al₂ ratio of at least 6,provided lactide yields above 20%, up to about 70%.

The results of the various experiments are summarized in Table 1. It isnoted that for some zeolites, the framework Si/Al₂ ratio, may differfrom the bulk Si/Al₂ ratio. For all zeolites, the framework Si/Al₂ ratiois provided, as this is most relevant ratio for the catalysis. For somezeolites, the bulk Si/Al₂ ratio is also provided (between brackets).

TABLE 1 Number of interconnected Si/Al₂ LA Lactide Catalyst non parallelframework conversion yield Name Topology Ring size channel systems ratio(%) (%) H-FER* FER 10-8  two systems (with 25 78.0* 25.1* one systemwith 10-membered ring channels) H-MCM-22 MWW 10-10 two systems with 2584.7 31.4 10-membered ring channels LaY FAU 12-12-12 three systems with5.2 72.4 10.5 12-membered ring channels LaX FAU 12-12-12 three systemswith 2.4 85.5 12 12-membered ring channels H-MOR MOR 12-8  Channelsystems 22 71.5 18.4 not interconnected (Parallel channel system with12- and 8-membered rings) H-BEA BEA 12-12-12 three systems with 150 98.059.1 12-membered ring channels H-BEA BEA 12-12-12 three systems with 2594.4 67.9 12-membered ring channels HCBV 780 FAU 12-12-12 three systemswith 75 (74)   93.3 45.7 12-membered ring channels HCBV 760 FAU 12-12-12three systems with 60 (54.6) 97.0 52.4 12-membered ring channels HCBV600 FAU 12-12-12 three systems with 19 (5.6)  89.9 53.4 12-membered ringchannels HCBV 500 FAU 12-12-12 three systems with 9 (5.2) 85.7 21.212-membered ring channels H-ZSM 5 MFI 10-10 two systems with 280 56.220.3 10-membered ring channels H-ZSM 5 MFI 10-10 two systems with 16088.5 52.6 10-membered ring channels H-ZSM 5 MFI 10-10 two systems with80 78.8 42.7 10-membered ring channels H-ZSM 5 MFI 10-10 two systemswith 50 71.5 27.8 10-membered ring channels H-ZSM 5 MFI 10-10 twosystems with 23 61.1 9.4 10-membered ring channels H₂SO₄ na na na na 1008.1 Amberlyst 15 na na na na 94.4 20.2 No catalyst na na na na 52.1 8.1na: not applicable *starting solution prepared by mixing 1.65 g of 50 wt% L-LA (aqueous) with 10 ml toluene From the amount of lactide obtained,the total (dry) weight of zeolite used, and the acid site density of thezeolite in each experiment, it is possible to calculate the rate oflactide formation per acid site for each zeolite. The acid site densitycan be estimated by assuming that each framework Al atom of the zeolitecorresponds with an acid site. These estimated values generallycorrespond well with values for acidity as determined via pyridinesorption.

FIG. 2 A shows the rate of lactide production per acid site (ascalculated from the Si/Al₂ ratio) per hour for ZSM-5 and H-BEA zeolites,with varying Si/Al₂ ratios. For the all zeolites except H-BEA (150), therate was calculated taking into account the amount of lactide formedafter 3 hours. For the H-BEA zeolite with a Si/Al₂ ratio of 150 (H-BEA(150)), the rate was calculated taking into account the amount oflactide formed after 1.5 hours, because this catalyst proved to besignificantly faster than the others. Taking this into account, it isclear that the H-BEA (150) zeolite provides the highest rate per acidsite.

The rate of lactide formation per gram of zeolite is a suitableindicator for the suitability of the zeolites for the catalysis oflactide formation (or cyclic ester formation in general). FIG. 2 B showsthe rate of lactide production per gram zeolite per hour for the samezeolites as in FIG. 2 A. It is clear that although, for example, theZSM-5 (160) zeolite provides a faster rate per acid site than the H-BEA(25) zeolite, the latter still provides a faster rate per gram ofcatalyst.

Degree of Polymerization

Via HPLC analysis of the reaction products, it was found that theaverage degree of polymerization (DP) of the formed oligomers using thezeolite catalysts was typically smaller than the DP of the formedoligomers using the reference catalysts (Amberlyst and sulfuric acid).Table 2 shows the reaction products and the average DP of the oligomersobtained with four different catalysts, under similar reactionconditions (oil bath of 130° C.; 3 h; 1 g L-LA 90 wt % (aqueous) in 10mL toluene). The results indicate a much higher average DP of theoligomers using the Amberlyst and sulfuric acid catalysts, compared tothe zeolite catalysts. This is an additional benefit of using zeolitesas catalysts, byproduct formation is less pronounced and the averagelength of byproduct oligomers is very small, rendering them watersoluble. Such small oligomers obtained with zeolite catalysts are moresuitable for re-introduction in a further cycle, for instance via simplehydrolysis into lactic acid, than the longer oligomers obtained with thereference catalysts. Longer oligomers (DP>5-6) typically tend to part inthe organic phase, which complicates the separation of the cyclic estersand the byproducts.

TABLE 2 Amount of dry LA Lactide Oligomers Catalyst catalyst (g) (%) (%)(%) DP Sulfuric acid 0.031 0.5 6.9 92.6 8.3 Amberlyst 15 0.5 2 20.2 77.810 wet H-BEA 0.45 5.6 67.9 26.5 3.4 (Si/Al2 = 25) ZSM-5 0.45 6.1 64.329.7 3.4 (Si/Al2 = 80) ZSM-5 0.475 11.5 52.6 35.9 3.4 (Si/Al2 = 160)

Regeneration of the Catalyst

The possibility to re-use the catalyst was assessed in a series of fouridentical runs with re-use of the catalyst. The reaction conditionswere: oil bath of 130° C.; 7 h reaction time; 1.65 g 50 wt % L-LA(aqueous) in 10 ml toluene; 0.5 g H-BEA (Si/Al2=25). A Dean-Stark trapwas used for removal of water from the reaction mixture. The samecatalyst was re-used in the four successive runs. After the first run,the catalyst was re-used in the next run after filtration and drying atroom temperature. This was repeated for the third run. After the thirdrun, the catalyst was calcined in air at 450° C. for 12 hours, using atemperature ramp of 3° C. per minute. The results of the reactions aresummarized in Table 3.

The results indicate that the re-use of the catalyst in runs 2 and 3leads to a small decrease of the lactide formation. However, aftercalcination of the catalyst (run 4), a similar lactide formation isobtained as in the initial run 1. This shows that the catalyst may befully regenerated via calcination.

TABLE 3 Lactide Oligomers Lactic acid Run (%) %) (%) 1 75.1 23.2 1.7 270.1 26.3 3.6 3 64.8 27.9 7.4 4 75.8 23.3 1.0

Influence of the Solvent

The influence of the solvent on the lactic acid yield was assessed in aseries of reactions under identical conditions, except for the solvent.The reaction conditions are: 3 h reaction time; 1 g 90 wt % L-LA(aqueous) in 10 ml solvent; 0.5 g H-BEA (Si/Al₂₌₂₅). The reaction wastypically performed at the boiling point of the reaction mixture. ADean-Stark trap was used for removal of water from the reaction mixture.

The tested solvents (and their boiling points) were as follows:cyclohexane (81° C.), toluene (111° C.), ethylbenzene (136° C.),p-xylene (138° C.), m-xylene (139° C.), o-xylene (143° C.), anisol (154°C.), propylbenzene (158° C.), mesitylene (163° C.). The results of thereactions are plotted in FIG. 3. Accordingly, in some embodiments,suitable solvents at atmospheric pressure can be solvents allowing for areaction temperature above 81° C. but below 163° C., under conditions ofwater removal.

Example 2 Preparation of Symmetric Cyclic Esters Other than Lactide3,6-diethyl-1,4-dioxane-2,5-dione: (ethyl glycolide)

3,6-diethyl-1,4-dioxane-2,5-dione was prepared using 2-hydroxybutanoicacid (2-HBA) in 10 ml of o-xylene, in the presence of H-BEA, at an oilbath temperature of 170° C. A Dean-Stark trap was used for removal ofwater from the reaction mixture. The reactants and results are shown inTable 4, as determined by ¹H-NMR and gas chromatography (GC).

TABLE 4 Reac- Yield Yield Hydroxycarboxylic tion Cyclic (NMR) (GC) acidand amount H-BEA time ester % % Racemic H-BEA 3 h R,S and 45.4 46.92-hydroxybutanoic (Si/Al₂: 25) meso acid 0.5g 0.25 g (R)- H-BEA 1 h R,R56.8 nd 2-hydroxybutanoic (Si/Al₂: 25) acid 0.5 g 0.25 g (S)- H-BEA 1 hS,S 75.5 77.6 2-hydroxybutanoic (Si/Al₂: 150) acid 0.5 g 0.25 g nd: notdetermined

The results indicate that H-BEA is a suitable catalyst for theproduction of racemic or enantiomerically pure ethyl glycolide. Thezeolite with a Si/Al₂ ratio of 150 appears to be faster and moreselective. This was confirmed in further experiments, wherein H-BEA withSi/Al₂ ratios of 25 and 150 were tested for the production of R,R-ethylglycolide (FIG. 4; Reaction conditions: 0.5 g (R)-2-HBA; 0.25 g H-BEA;10 mL o-xylene; oil bath temperature of 170° C.; reaction time 1 h or 3h).

The produced ethyl glycolide could then be used to prepare poly(ethylglycolide) by ring opening polymerization (as described in Yin et al.(1999), Macromolecules, 32(23), 7711-7718) in the presence oftin-2-octanoate/neopentylalcohol (ratio catalyst:initiator 1:1), theratio monomer:catalyst was 100:1. The polymerization was performed at130° C. under light pressure of helium for 2.5 hour. After 2.5 h ofreaction a polymer of about 3000 g/mol was obtained.

3,6-divinyl-1,4-dioxane-2,5-dione

3,6-divinyl-1,4-dioxane-2,5-dione was prepared using 0.5 g(DL)2-hydroxy-3-butenoic acid in 10 ml of toluene, in the presence ofH-BEA (Si/Al₂: 25) (0.25 g) at an oil bath temperature of 130° C. ADean-Stark trap was used for removal of water from the reaction mixture.After 24 h of reaction, the yield was 24%.

3,6-dibutyl-1,4-dioxane-2,5-dione

3,6-dibutyl-1,4-dioxane-2,5-dione was prepared using 2-hydroxyhexanoicacid (0.5 g) in 10 ml of o-xylene, in the presence of H-BEA (Si/Al₂: 25)(0.25 g), at an oil bath temperature of 170° C. A Dean-Stark trap wasused for removal of water from the reaction mixture. After 3 h ofreaction, the yield was 9.5% as measured by NMR.

1,4-dioxane-2,5-dione (glycolide)

Glycolide was prepared using 70 wt % glycolic acid (aqueous) (1 g) in 10ml of toluene, in the presence of ZSM-5 (Si/Al₂: 160) (0.5 g), using anoil bath at a temperature of 170° C. A Dean-Stark trap was used forremoval of water from the reaction mixture. After 3 h of reaction, theyield was 31.1%.

Example 3 Preparation of Asymmetric Cyclic Esters

Equimolar amounts of D-2-hydroxybutyric acid and L-lactic acid weremixed in o-xylene and reacted in the presence of H-BEA (Si/Al₂: 25)(0.25 g) at an oil bath temperature of 170° C. A Dean-Stark trap wasused for removal of water from the reaction mixture. After 3 h ofreaction, the products as listed in Table 5 were obtained, as measuredby H¹-NMR and confirmed by GC.

TABLE 5 Yield GC Yield NMR Cyclic ester (%) (%) L-L-lactide 29 22D-D-ethyl glycolide 23.8 21.5 Asymmetric meso cyclic 46.5 42 ester

Example 4 Caprolactone

Caprolactone was prepared using 6-hydroxyhexanoic acid (0.5 g) in 10 mlof toluene, in the presence of H-BEA (Si/Al₂: 25) (0.2 g), using an oilbath at a temperature of 130° C. A Dean-Stark trap was used for removalof water from the reaction mixture. After 3 h of reaction, the yield was99% as measured by GC.

Example 5 Caprolactam

Caprolactam was prepared using 6-aminohexanoic acid (1 g) in 10 ml oftoluene, in the presence of H-BEA (Si/Al₂: 25) (0.5 g), using an oilbath at a temperature of 130° C. A Dean-Stark trap was used for removalof water from the reaction mixture. After 3 h of reaction, the yield was5%.

1-15. (canceled)
 16. A process for preparing a cyclic ester or a cyclicamide, comprising: contacting at least one hydroxycarboxylic acid and/orat least one amino-carboxylic acid; or an ester, or salt thereof;wherein said hydroxycarboxylic acid is a 2-hydroxycarboxylic acid, or a6-hydroxycarboxylic acid; and wherein said amino carboxylic acid is a2-amino-carboxylic acid or a 6-amino-carboxylic acid; with at least oneacidic zeolite comprising: two or three interconnected and non-parallelchannel systems, wherein at least one of said channel systems comprises10- or more-membered ring channels; and a framework Si/X₂ ratio of atleast 24 as measured by NMR; or three interconnected and non-parallelchannel systems, wherein at least two of said channel systems comprise10- or more-membered ring channels; and a framework Si/X₂ ratio of atleast 8 as measured by NMR; wherein each X is Al or B, wherein theprocess is performed at a pressure between 0.5 and 20 bar, and whereinsaid cyclic ester or cyclic amide is a compound of formula (II):

wherein Z¹ is O or NH, wherein Z² is O or NH; and wherein R¹ and R² areeach independently hydrogen or a group selected from C₁₋₆alkyl;C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene; C₆₋₁₀arylC₁₋₆alkylene;or C₂₋₆alkynyl; each group being optionally substituted by one or moresubstituents selected from C₁₋₆alkyl, C₁₋₆alkyloxy.
 17. The processaccording to claim 16, wherein said cyclic ester is a compound offormula (IIa):

wherein R¹ and R² are each independently hydrogen or a group selectedfrom C₁₋₆alkyl; C₂₋₆alkenyl; C₆₋₁₀aryl; C₁₋₁₀alkylC₆₋₁₀arylene;C₆₋₁₀arylC₁₋₆alkylene; or C₂₋₆alkynyl; each group being optionallysubstituted by one or more substituents selected from C₁₋₆alkyl,C₁₋₆alkyloxy.
 18. The process according to claim 16, wherein said atleast one hydroxycarboxylic acid is selected from the group comprisinglactic acid, 2-hydroxybutanoic acid, 2-hydroxy-3-butenoic acid,2-hydroxyhexanoic acid, 6-hydroxyhexanoic acid, and glycolic acid. 19.The process according to claim 16, wherein said cyclic ester is lactideand said at least one hydroxycarboxylic acid is lactic acid.
 20. Theprocess according to claim 16, wherein said cyclic ester is L-L-lactideand said at least one hydroxycarboxylic acid is L-lactic acid.
 21. Theprocess according to claim 16, wherein said cyclic ester is D-D-lactideand said at least one hydroxycarboxylic acid is D-lactic acid.
 22. Theprocess according to claim 16, wherein at least one of saidinterconnected and non-parallel channel systems comprises 12- or moremembered ring channels.
 23. The process according to claim 16, whereinsaid zeolite has a Brønsted acid density between 0.05 and 6.5 mmol/g dryweight.
 24. The process according to claim 16, wherein said zeolitecomprises a topology selected from the group comprising BEA, MFI, FAU,MEL, FER, and MWW.
 25. The process according to claim 16, wherein saidzeolite comprises a BEA topology.
 26. The process according to claim 16,wherein X is Al.
 27. The process according to claim 16, wherein saidzeolite comprises at least three interconnecting and non-parallelchannel systems.
 28. The process according to claim 16, wherein saidhydroxycarboxylic acid and/or aminocarboxylic acid is provided in acomposition comprising said hydroxycarboxylic acid and/oramino-carboxylic acid in a concentration of at least 1 wt % based on atotal weight of the composition.
 29. The process according to claim 16,wherein said process is performed under conditions of water removal. 30.The process according to claim 29, wherein said water removal isperformed via azeotropic distillation.